1. Field of the Invention
The present invention relates to high-capacity cryosorption vacuum pumps for high-energy chemical lasers and more particularly to a vacuum pump system for a Chemical Oxygen-Iodine Laser (COIL).
2. Description of the Related Art
While not limited thereto, the present invention is particularly adapted for use with a high-power Chemical Oxygen-Iodine Laser intended for military applications. In chemical lasers, chemical reactions are used to produce exited atoms or molecules in a flow of suitable mixture of rarefied gases. Gas containing excited species is flowed through a laser cavity where optical energy is extracted from the excited species by means of an optical resonator. Required flow throughput and pressure are produced by vacuum pumps which draw the gas mixture through the laser cavity. High-energy chemical lasers for military applications often produce hundreds of kilowatts of optical power. The corresponding gas throughput in the range of 10-100 Torr pressure requires vacuum pumps with pumping speeds on the order of several hundred thousand liters per second. Military applications for high-power chemical lasers include tactical air defense which necessitates deployment of laser weapons in forward positions on the battlefield. Such laser weapons must be transportable and, therefore, of limited size and weight. In addition, the laser weapon should be concealable and undetectable by the enemy.
High-energy chemical lasers can be classified as either 1) hydrogen-halide or 2) Chemical Oxygen-Iodine Laser (COIL). Hydrogen-halide lasers typically involve a reaction of hydrogen and/or deuterium with fluorine, chlorine, bromine or iodine in diluent gases of nitrogen, helium, or alike, to produce hydrogen and/or deuterium halide molecules in excited vibrational states from which laser energy can be extracted. Exhaust from the laser cavity of a hydrogen-halide laser is typically a mixture of gases at high temperature (up to 1000 degrees Centigrade) including HF (and/or DF), N.sub.2, and possibly small amounts of H.sub.2 (and/or D.sub.2), O.sub.2 and H.sub.2 O.
On the other hand, COIL lasers typically involve reaction of chlorine in diluent gases such as nitrogen or helium, with aqueous solution of alkaline hydrogen peroxide to produce intermediate excited specie known as singlet delta oxygen. Singlet delta oxygen is subsequently mixed with iodine vapor to generate iodine atoms in electronically excited state and suitable for extraction of laser energy. Exhaust from a COIL laser cavity is typically a mixture of gases at near ambient temperature including nitrogen or helium and oxygen with small amounts of chlorine, iodine, and water.
In laboratory practice, mechanical vacuum pumps, roots blowers, and steam ejectors are used to maintain flow conditions in chemical lasers. Gas exhausted from chemical lasers often contains hazardous materials such as iodine, chlorine, fluorine, hydrogen and hydrogen fluoride. In order to prevent the release of such materials into atmosphere the laser exhaust gas must be scrubbed. Mechanical pumps with their concomitant scrubbers are too heavy and bulky for use in a transportable, field-deployable high-power laser. To overcome the size and volume limitations of mechanical pumps, Naismith et al, in U.S. Pat. No. 3,879,680 proposed a decontamination-capable combustion-driven ejector for a hydrogen fluoride laser. However, ejectors, although smaller in size and weight than corresponding mechanical pumps, are still excessively large and heavy for use in a transportable COIL where low cavity pressure necessitates two-stage pumping. Furthermore, operation of ejectors is typically accompanied by acoustic noise and liberation of large amounts of hot gases and/or steam into the atmosphere which significantly reduce concealment and increase detectability of the high-power laser weapon.
A vacuum pump capable of pumping and safely containing exhaust from a hydrogen fluoride laser has been disclosed by Ogren et al. in the U.S. Pat. No. 3,992,685. Pumping action here is produced by chemically reacting laser cavity exhaust gases with titanium, zirconium, and other reactive metals at elevated temperature. The laser exhaust is thus safely contained within the vacuum pump envelope. A refinement of Ogren's device and process was disclosed by Blumenthal et al. in the U.S. Pat. No. 4,514,698 where pumping action is produced by a combination of condensation (to remove HF and/or DF), chemical reaction with Ti, Zr, etc. (to remove O.sub.2, H.sub.2 and/or D.sub.2) and cryogenic adsorption to remove nitrogen. A considerable disadvantage of Ogren's and Blumenthal's processes is the need to separately remove constituents from the flow in several steps some of which require high temperature reactions with metals. Since some of the reactions with metals are difficult to reverse it can be deemed that neither Ogren's nor Blumenthal's apparatus could be easily regenerated. Blumenthal describes cryosorption of nitrogen only as a part of the multi-step pumping process whereas oxygen is pumped by reaction with hot metals. In summary, the inventions of Ogren and Blumenthal are very specific for use with the hydrogen-halide laser and no reference is made to show how they may be adapted to COIL.
Cryosorption pumps of various designs have been used in the vacuum industry for many years (Cryopumping Theory and Practice, Chapter 5, by Rene A. Haefer, Claredon Press, Oxford, UK, 1989). Sorption pumps function by the physical adsorption of gases at the surface of molecular sieves or other sorption material (e.g. activated Al.sub.2 O.sub.3 or charcoal). Such materials have an extraordinarily large specific surface area per unit of mass (hundreds of m.sup.2 /gram). Correspondingly, the capability of gas adsorption is considerable, up to 200 milligrams of nitrogen per gram of synthetic zeolite (Linde 4A, manufactured by Union Carbide Corp., Danbury, Conn.) at the temperature of liquid nitrogen (77 degrees Kelvin). A variety of natural and synthetic zeolites are now commercially available. Sorption capacity of zeolites (maximum amount of gas that can be stored) is highly dependent on zeolite temperature and pressure of gas above the sorption surface. In particular, at a constant pressure, the sorption capacity increases with decreasing temperature while at a constant temperature the sorption capacity decreases with decreasing pressure. For example, at a pressure of 10 Torr, changing the temperature from 293 degrees Kelvin to 77 degrees Kelvin increases the capacity of zeolite (e.g. Linde 4A) to sorb to nitrogen more than 200 times. Furthermore, during the sorption process the sorption effect decreases with increased coverage of the sorption sites.
The sorption capacity of zeolites is also highly dependent on the gas to be pumped. In general, light inert gases are hardly pumped at all. For example, the capability of synthetic zeolite Linde 4A to pump helium or neon at a temperature of 80 degrees Kelvin is several orders of magnitude lower than for COIL laser gases such as oxygen and nitrogen.
In general, cryosorption vacuum pumps can be classified as roughing and hard vacuum type. Cryosorption vacuum pumps for roughing applications are capable of evacuating vacuum chambers from atmospheric pressure down to a fraction of a Torr. These devices are usually quite simple in construction, comprising a metal flask containing zeolite. Vacuum suction is obtained at the flask throat as the flask is immersed into a bath of liquid nitrogen. The pumping process exerts a heat load to the zeolite. The heat load is due to a change in enthalpy of the gas as it is being cooled to the temperature of the zeolite and release of the heat of sorption. Since the zeolite can adsorb atmospheric oxygen and nitrogen only when cold, the pumping speed of a zeolite roughing pump depends on its effectiveness to reject the heat load to the liquid nitrogen. The problem of maintaining the zeolite at low temperature is further exasperated by zeolite's poor thermal conductivity. Zeolite roughing pumps are normally used in applications where time is not critical. Pump-down times on the order of 10-60 minutes are acceptable.
Cryosorption vacuum pumps for hard vacuum applications (below 10.sup.-3 Torr) normally encounter a lower head load than roughing pumps. This is both due to the reduced gas density and a refrigerated baffle (usually a chevron style) which is normally located at the intake to the pump and cools the incoming molecular flow of gas. Various design of such pumps have been disclosed in prior art, for example by Thibault et al. in the U.S. Pat. No. 3,668,881; Lessard et al. in the U.S. Pat. Nos. 4,494,381 and 4,718,241; Sukenobu in the U.S. Pat. No. 4,607,493, and Larin et al. in the U.S. Pat. Nos. 4,99,369, 5,005,363 and 5,014,517. It should also be noted that cryosorption vacuum pumps for hard vacuum are not suitable for operation at high pressures (significantly above 10.sup.-3 Torr) due to their inability to reject concomitant increase in heat load.
A hydrogen halide laser entirely pumped by cryosorption has been described by Newton et al. in the article entitled: "Cryosorption-pumped cw chemical laser" which was published in the Applied Physics Letters vol. 33(1), on Jul. 1, 1978. Newton et al. used a commercially available zeolite sorption pump cooled by liquid nitrogen to operate a small (200-300 miliwatt) hydrogen halide laser at cavity pressures of a few Torr and flow rates of several milimoles per second for periods of up to 6 hours. Because of its low flow rates, Newton's cryosorption pump has not experienced problems with rejection of heat of adsorption. However, Newton's concept is not scalable to a high-power chemical laser with its concomitant high flowrates.
In summary, a suitable cryosorption vacuum pump system for a COIL requires the capability of handling relatively short (about 100 second) duration gas flow with a throughput on the order of 10-100 mol/s at about 10 to 30 Torr pressure. Gases to be pumped are expected to be at near ambient temperature (300 to 400 degrees Kelvin), possibly moist (containing water vapor and possibly particulates), and contain corrosive and hazardous materials such as iodine and chlorine. The cryosorption vacuum pump system should be light-weight, compact, economical in refrigerant use, environmentally safe, and have a short regeneration time. Devices and methods disclosed in the prior art cannot meet these requirements simultaneously. A new cryosorption vacuum pump system, one specific for the needs of the chemical oxygen-iodine laser, is needed.